1. Field of the Invention
This invention relates to an article having a metallized surface, e.g. a substrate having an electrically conductive surface with a frequency selective surface designed to pass selected frequencies of the electromagnetic spectrum, and more particularly, to a transparency, e.g. an automotive windshield having an electrically heatable coating and a frequency selective surface formed in the coating, the frequency selective surface designed to pass selected radio frequencies with little or no attenuation while reducing the intensity of hot and cold spots around the a frequency selective surface.
2. Discussion of the Technology
Automotive heatable windshields, e.g. of the type disclosed in U.S. Pat. No. 4,820,902, include two glass sheets laminated together by a plastic interlayer, usually a sheet of polyvinyl butyral (“PVB”). A pair of spaced bus bars between the glass sheets are in electrical contact with an electrically conductive member, and each bus bar is electrically accessible by an external lead to pass current from a power source through the bus bars and the conductive member to electrically heat the conductive member and conductively heat the inner and outer surfaces of the windshield. The heated windshield surfaces attain a temperature sufficient to remove fog and melt snow and ice.
For a general discussion of heatable windshields reference can be had to U.S. Pat. Nos. 3,789,191; 3,789,192; 3,790,752; 3,794,809; 4,543,466, and 5,213,828.
The electrically conductive member of heatable windshields is usually a sputtered electrically conductive coating applied to a major surface of one of the glass sheets and usually includes one or more conductive films, e.g. silver films, with each of the conductive films between a pair of dielectric films. The silver film in addition to passing current also reflects infrared energy. When there are no facilities to move current through the conductive coating, the coating functions as a solar control coating; when there are facilities to move current through, and current moves through the coating, the coating is heated and further functions as a heatable coating. Coatings of the type discussed above are disclosed in European Patent Application No. 00939609.4.
The electrically conductive and/or solar control coatings are usually provided with an area or aperture to pass frequencies of the electromagnetic spectrum. More particularly, modern day communication is heavily dependent on the transmission of RF signals to pass information into the interior of vehicles, e.g. to pass signals to AM/FM radios, CB radios, cellular phones, and/or to obtain information from the interior of the vehicle, e.g. to obtain information for global positioning systems, automatic toll collection transponders, radar systems, and various other satellite systems operated using RF communication, and/or to energize systems, e.g. rain sensors and lighting circuits. Accordingly, there is a need to adapt the transparencies such as automotive windshields to permit the transmission of RF signals. One adaptation is to leave an uncoated area in the coating. For example, the 1990–1995 Chevrolet Corvette ZR-1 was offered with windshield having a solar control coating. An uncoated area was provided at the bottom center of the windshield. The manual provided by the car manufacturer informed the car user that the specially-treated windshield would decrease the effective distance of some add-on electronic devices, such as garage door openers, and provided instructions that if such a device is used it should be aimed through the uncoated area.
In the instance when the coating functions only as a solar control coating, the uncoated area has a higher visible light transmission and is noticeable, i.e. aesthetically unacceptable. In the instance when the coating is an electrically heatable coating, the uncoated area is not heated, leaving ice and snow on the windshield in the region of the uncoated area and creating hot and cold spots around the perimeter of the uncoated area due to localized high current or low current regions.
Since 1990, improvements have been made to overcome the drawbacks associated with having an uncoated aperture in heatable and/or solar control coatings. One of the improvements is to provide a frequency selective surface, i.e. a pattern in the communication window that has selected portions of the coating removed to pass electromagnetic waves having at least one predetermined operating frequency or frequency range and block, to the extent any metal or insulating sheet blocks, other frequencies. One such frequency selective surface is a patch array. Frequency selective surfaces require removal of a small percentage of the coating thereby maintaining the aesthetics of the windshield. Frequency selective surfaces are discussed in U.S. Pat. Nos. 3,633,206; 3,789,404; 3,961,333; 3,975,738; 4,126,866; 4,301,456; 4,479,131; 4,656,487; 4,851,858; 5,208,603; 5,311,202; 5,364,685, and 5,528,249 and WO 96/31918 and in Krause “Antennas”, “Slot, Horn, and Complementary Antennas” pp 353–371, First Edition, McGraw-Hill, 1950.
Although the presently available frequency selective surfaces reduce the difference between the visible light transmission of the coating and the aperture making the windshield aesthetically acceptable, they have not eliminated the drawbacks of using the frequency selective surface with heatable coatings. More particularly, frequency selective surfaces such as patch arrays are provided in the aperture spaced from the coating and do not provide for current to move through a frequency selective surface in the aperture. When the coating is energized to remove snow and ice, the frequency selective surface is not heated, leaving ice and snow on the windshield in the region of the a frequency selective surface and creating hot and cold spots around the perimeter of the aperture.
As can be appreciated by those skilled in the art, it would be advantageous to provide frequency selective surfaces that can be used with heatable coatings and do not have the drawbacks of the presently available frequency selective surfaces and that pass radio waves of selected frequencies and polarization with little attenuation.